In Chapter 6, we have been introduced to the concept of entailment as a kind of semantic relationships among sentences. It refers to the relationship in which the truth of one sentence guarantees the truth of another sentence. We say that sentence X entails sentence Y if when X is true, Y is necessarily true in all situations; that is, it is impossible to assert X but deny Y (see more detail of entailment in Section 3, Chapter 6). For example, the first sentence of the pair in (20) entails the second one